Heat exchange apparatus



Jan. 22, 1963 F. GIAUQUE 3,074,479

HEAT EXCHANGE APPARATUS Filed Jan. 15, 1960 2 Sheets-Sheet 1 INl/ENTORLOU/S F. G/AUQUE Jan. 22, 1963 1.. F. GIAUQUE HEAT EXCHANGE APPARATUS 2Sheets-Sheet 2 Filed Jan. 15, 1960 FIG. 2

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INVENTO/P LOU/S E G/AUQUE ATTORNEYS Patented Jan. 22, 1963 3,074,47 HEATEXCHANGE APPARATUS Louis F. Giauque, 50 Dominion Ave.,

Kapuskasing, Ontario, Canada Filed Jan. 15, 1960, Ser. No. 2,645 3Claims. (Cl. 165-110) This invention relates to a heat exchangeapparatus. The invention is concerned with apparatus for passing steamin heat exchange relation with air having an 111- itial temperaturebelow the freezing point of water; such apparatus is commonly calledblast coils or heater colls.

It is known to employ steam to heat air for use in the lieating ofbuildings such as factories, aircraft hangars, etc. One form ofapparatus used for heating the air comprises two or more rows of heatexchanger tubes arranged one behind the other; steam is passed throughthe tubes from a common header, the air is propelled across the exteriorof the tubes by means of a fan, and the condensate is collected in acommon header. It has been found that, under certain conditions of airflow when the air is initially at a temperature below the freezing pointof water, the condensate which forms in the row of tubes With which theair first comes into contact freezes and bursts the tubes. A row oftubes with which the air first comes into contact is hereinafter calleda first row. Air at a sufficiently low temperature to cause bursting ofthe tubes as described above is commonly encountered in the winter in,for example, the north-westerly parts of the United States and most ofCanada and the invention is particularly concerned with providing heatexchange apparatus which may be used in such northerly latitudes andwhich prevents, or at least minimizes, the freezing of condensate in thetubes.

Although the makers of heat exchanger tubes realize that a freezingproblem exists in connection with heating very cold air, they have beenunable to provide a satisfactory solution. So called non-freezing orsteam distributing heater tubes which are presently on the marketcomprise an outer finned tube within which is concentrically arranged aninner tube and a clearance is provided between the outer surface of theinner tube and the inner surface of the outer tube. The wall of theinner tube is perforated all along its length and steam is supplied tothe inner tube and passes through the perforations into the clearancebetween the tubes. This construction is intended to prevent thecondensate from fi'eezing in the tubes but it is not effective for thispurpose in apparatus having more than one row of tubes connected to acommon condensate header. Moreover, such non-freezing tubes costapproximately one third more than conventional heat exchanger tubes.

Freezing may be prevented by using excessive amounts of steam, e.g. bypassing steam through the tubes without restriction, but this is notpractical in the majority of installations where steam is an expensivecommodity.

I have now found a solution to the problem of the freezing of condensatein the tubes of the first row in a double or multiple row heatexchanger. My solution does not require the use of excess amounts ofsteam and is to provide means to eliminate the leg of condensate which Ihave found normally builds up and freezes in each tube of the first rowwhen this row is connected with a subsequent row or rows to a commoncondensate header. I achieve this effect by allowing the pressure dropof the steam across the tubes of the first row to be independent of thepressure drop across the tubes of the other row or rows.

I have tested my invention in practice and found that it is operative.For example, when a particular installation of two rows of tubesarranged one behind the other is used in accordance with my invention,no condensate freezes in the tubes even when the air is initially at 30"F. and passes over the tubes at 750 feet per minute and when the steampressure applied to the tubes is as low at 7 p.s.i. Previously, when theinstallation was used in a conventional manner, condensate froze in thetubes when they were used to heat air passing over them at 750 feet perminute at an initial temperature l0 F. and with a steam pressure of 15p.s.i. across the tubes.

I have developed the following theory which I believe explains themechanism of my invention. It is apparent that, in a two row heatexchanger with the tube rows arranged one behind the other in thedirection of air flow, the air passing across the tubes in the first rowis colder than the air passing across the tubes in the second row. Ittherefore follows that the temperature difference between the steam inthe tubes of the first row and the air passing over the tubes of thefirst row is greater than the temperature difference between the steamin the second row and the air passing over the tubes of the second row.For this reason, the air extracts more heat from the steam flowingthrough the tubes of the first row than from the steam flowing throughthe tubes in the second row. It follows that more steam is required topass through the tubes of the first row than through the tubes of thesecond row and that therefore the pressure drop across the tubes in thefirst row is greater than the pressure drop across the tubes in thesecond row since each pressure drop is proportional to the square of thequantity of steam flowing through the tubes of the row.

When the tubes in both rows are connected to a common condensate header,I believe that it is this difference in pressure drop across the tubesof the various rows that maintains a leg of condensate in a lowerportion of each tube of the first row. I have found that by isolatingthe lower or condensate ends of the tubes of the first row from thecorresponding ends of the tubes of the second row and by connecting thetubes of each row to a separate steam trap, I have eliminated thecondensate leg which has hitherto built up in each tube of the first rowand I have obviated the freezing and bursting of the tubes of the firstrow. I believe that the reason Why this arrangement obviates thecondensation leg is that my arrangement allows the pressure drop acrosseach row to be independent of the pressure drop across the other row sothat there is no excess pressure at the condensate ends of the tubes ofthe first row which tends to hold a leg of condensate in the tubes ofthe first row. On the other hand, when the tubes of both rows areconnected between a common steam header and a common condensate header,I believe that the pressure in the common condensate header isdetermined by the amount of steam flowing through the tubes of thesecond row, i.e. by the pressure drop across the tubes of the secondrow, and is sufiicient to hold, in the tubes in the first row, legs ofcondensate which then freeze and burst the tubes.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIGURE 1 is a diagram showing the various components of the heatexchange apparatus according to the invention,

FIGURE 2 is a perspective view, partly broken away, of the heatexchanger tubes and traps forming part of the apparatus shown in FIGURE1,

FIGURE 3 is a broken away perspective view of one of the traps shown inFIGURES 1 and 2, and

FIGURE 4 is a transverse cross section of a heat exchanger coil of thetype used heretofore in installations for heating air by steam.

3 Referring now to the drawings, and particularly to FIGURE 1, there isshown a duct along which cold air is drawn by a fan 11 driven by anelectric motor 12 supported Within the duct by struts 13. The cold airis propelled by the fan through first and second rows of heat exchangertubes indicated at 14 and 15 respectively. At their upper ends the tubesof the rows 14 and 15 are connected to a common steam header 16 which issupplied With steam from a source or boiler 17 by means of a steam pipe18. The bottom ends of the tubes in the first row 14 are connected to afirst condensate header 19 which is connected by first conduit means 20to a first steam trap 21. Similarly, the tubes of the second row 15 areconnected to a condensate header 22 which is connected by second conduitmeans 23 to a second steam trap 24. The .traps 21 and 24 discharge theconden'sate to a hot well or condensate sump (not shown) through a pipe25.

Referring now to FIGURE 2, the arrangement of the rows of heat exchangertubes is shown in more detail.

The rows of tubes are held in a rectangular frame made up ofchannel-section side members 26 and channel-section upper and lowercross members 27. The ends of the cross members 27 are closed by ribs28. Secured to the underside of the upper cross member 27 and parallelthereto is the common steam header 16 which is of boxshapedcross-section and is provided with a steam inlet connection 29 securedto the steam pipe 18. The upper nected to the condensate headers 19, 22respectively at condensate connections intermediate the ends of thecondensa-te headers. Upper and lower bafiies 30a shield the steam header16 and the condensate headers 19 and 22 so that the air to be heated hasto pass through the finned tube section of the apparatus.

Both of the traps 21 and 24 are similar and the trap 24 has been shownin FIGURE 3 as illustrative of both traps. The trap comprises a hollowbody 31 having a steam and condensate inlet 32 and a condensate outlet-33; the condensate and steam passes through a strainer 34 afterleavingthe inlet 32. A float 35 is secured to an angle member 36 having sidearms 37 which are pivoted to the body 31 at 38. Also secured to theangle member 36 is a bimetallic element 39 which has secured to one endthereof upstanding lugs 40; pivoted between the lugs 40 is a'bar 41slotted at 42. Received in the slot 42 is a rod 43 having a ball 44 atits lower end and arranged to co-operate with a valve seat 45 at thelower end of an insert 46. The upper end of the rod 43 is provided withan adjusting nut 47 having a sleeve portion which is received in theslot 42. Access plugs 48 are provided in the body and a bracket 49 issecured to the float to control its lowermost position.

The trap is shown in a position it occupies when there is steam in thetrap but no condensate. The steam causes the bimetallic element to bendand to lift the rod so that the ball 44 comes into contact with the seat45 and prevents the escape of steam. When cooler condensate enters thetrap, the bimetallic element straightens, and the float 35 rises todischarge the condensate through the insert 46. Once the apparatus is insteady operation the float will operate to maintain a predeterminedlevel of condensate in the trap and will discharge the excess to the hotwell. It will be seen that the trap provides means to prevent the escapeof steam and to discharge condensate.

The operation of the apparatus is as follows: steam is supplied from thesteam supply 17 and passes through the steam pipe 18 to the common steamheader 16 so that it is supplied to all the tubes at the same pressure.The steam then passes through the tubes in the rows 14 and 15 and anysteam which is condensed in the tubes passes into the headers 19 and 22.The condensate delivered to the condensate header 19 passe along thefirst conduit means to the trap 21 and is discharged but the trapprevents escape of steam from the condensate header. Similarly,condensate delivered to the header 22 passes along the second conduit 23to the trap 24 where the condensate is discharged but the steam isprevented from escaping. The fan 11 propels air across the tubes so thatthe air passes first through the tubes in the first row 14 and thenthrough the tubes in the second row 15. The air extracts heat from thesteam in the tubes and leaves the tube assembly at a desiredtemperature.

The following exemplary calculation will show the temperatures,pressures and pressure drops across the tubes. In a particular examplethe length of the tubes is 121", they have an inside diameter of 0.527",an inside area of 0.2185 sq. in. and they are provided with an orificeat the tube entrance which is 0.375" in diameter. The steam supplied tothe steam header 16 is at a pressure of 15 psi. and at a temperature of250 F. The steam has a density of 0.0721 lb./ft. a specific volume of13.88 ft. /lb. and a latent heat of 946 B.t.u./lb. The air is arrangedto pass over the tubes at a speed of 700 feet/minute, and has an initialtemperature of 40 F., a density of 0.075 lb./ft. and a specific heat of0.24 B.t.u./lb./ F. From tables provided by the tube manufacture it maybe ascertained'that a row of thirty tubes each having a length of 121has a net face area of 35 sq. ft. and it follows therefore that the facearea of a single tube is 1.167 sq. ft. (The face area is the net area ofthe tube which faces the oncoming air stream and is the same for eachtube in each row.)

The manufacturer of the tubes also provides tables whereby thetemperature of the .air leaving a row of tubes may be calculated giventhe initial temperature of the entering air and the saturationtemperature of the steam. The formula is as follows:

T, T. T. T;

where T is the saturation temperature of the steam, T is the temperatureof the air entering a row and T is the temperature of the air leavingthe row. From tables provided by the manufacturer, for this particularexample, K=1.31 for one row of tubes and is 1.70 for two rows of tubes.

Using the above formula and the given values of K, it follows that thetemperature of the air leaving the first row of tubes 14 equals JfiifiLo 250 1.31 --28 F.

Following a similar calculation the temperature of the air leaving thesecond row of tubes equals 1.167X700X0.075 X0.24 (28+40) =14.7 68=999.6B.t.u./min.

Similarly, the heat transferred to the air from the steam by one tube ofthe first row plus the heat transferred to the air from one tube of thesecond row is 14.7X (79+40) 1749.3 B.t.u./min.

It follows that the heat transferred to the air from one tube of thesecond row is 749.7 B.t.u./min.

In order to supply 999.6 Btu/min. from each tube of the first row itfollows that BIL/H1111.

of steam must be passed through each tube of the first row, 946B.t.u./lb. being the latent heat of the steam. Similarly, the quantityof steam which must pass through each tube of the second row is =0.77slb./min.

This is assuming that no sensible heat is extracted from the condensate.This is not strictly accurate; obviously it is sensible heat taken fromthe condensate which causes the latter to freeze, but the amount ofsensible heat eX- tracted may be neglected in comparison with the amountof latent heat given up by the steam.

The velocity of the steam as it enters each tube of the first row,neglecting the efiect of the orifice, is given by the weight of thesteam flowing/min. through the tube multiplied by the specific volume ofthe steam and divided by the inside area of the tube. This is Similarly,the velocity of the steam as it enters each tube of the second row is0.778X 13.88X 144 WET- 8.2 ft./sec.

The pressure differential required to force steam through a tube may bestated in terms of the velocity pressure of the steam entering the tube.The velocity pressure may be calculated from the formula v :2gh where his the pressure head due to the velocity. To convert this into inches ofwater gauge v 12X steam density 64.4Xwater density of the steam passingthrough the second row of tubes equals water gauge.

I have found that the relationship between the pressure drop in a tubeand the tube length will not be linear but will follow some law such asmultiplied by the velocity head. This law is due to the fact that as thesteam passes through the tubes it continuously diminishes in quantitydue to condensation.

On the basis of experiment I believe the pressure drop to be given by alaw which is approximate to the form multiplied by the velocity head,where L and D are the tube lengths and inside diameter respectively. Forthe figures given above of a tube length of 121" and an inside diameterof 0.527" the law simplifies to 22.0 multiplied by the velocity head. Tothis I have added a loss due to the steam passing through the orifice atthe entrance to the tube. I have calculated that the loss due to theorifice is 0.2 multiplied by the velocity head and therefore thepressure loss coefiicient is 22.2. It follows that the pressure dropthrough a tube in the first row is 22.2 multiplied by the velocity headof the steam in inches of water gauge for that tube =5.40 22.2= in.water gauge. Similarly, the pressure drop in a tube of the second rowequals 3.02 22.2= 67 in. water gauge.

A pressure of 15 lb./sq. in. is approximately 415 inches water gauge andtherefore the pressure in the steam header 16 is 415 in. water gaugewhereas the pressure in the first condensate header 19 is 415120=295 in.water gauge and the pressure in the second condensate header 22 is4l567=348 in. water gauge. There is therefore a difference in pressurebetween the two condensate headers 19 and 22 of 53 in. of water gauge.It will be seen that, according to my invention, I permit the pressuredrop across each row of tubes to be independent of the pressure dropacross the other row and that I maintain the pressure drops at valuesdetermined by the quantity of steam condensed in the respective rows.Thus less steam is condensed in the second row than in the first row andtherefore the pressure drop across the first row is more than thepressure drop across the second row.

In conventional apparatus a common steam header and a common condensateheader are provided so that the condensate ends of the tubes of bothrows are in communication. I believe that the column of condensate whichI have found is held in the tubes of the first row is due to thedifferences in the pressure drops in the tubes and to the use of acommon condensate header.

Referring now to FIGURE 4 there is shown a conventional construction ofheater coil in which freezing readily occurs in the tubes of the firstrow. The tubes are held within a framework and are connected between acommon steam header 50 and a common condensate header 51. A first row oftubes is indicated at 52 and a second row of tubes at 53 and cold air ispassed through the tubes in the direction of the arrow X. Condensatedelivered to the common condensate header 51 leaves through a conduit 54and passes to a trap 55 and thence to a hot well (not shown) through apipe 56.

In this arrangement I believe that the pressure difference between thesteam header 50 and the condensate header 51 will be determined by thepressure drop across the second row of tubes 53. For a row of coils ofsimilar size to those discussed above, this pressure drop across thesecond row will be approximately that calculated above, i.e. 67 inchesof water gauge when the entering air has a temperature of 40 F. and isflowing at a velocity of 700 f.p.m. If the pressure in the condensateheader were more than 67" water gauge below the steam header pressure,then more steam would pass through the second row of tubes. However, nomore steam can pass through the tubes since the above calculations showthat the tubes have condensed all they can with a pressure drop of 67inches of water gauge and any further steam that is unable to passthrough the trap 55 would merely increase the pressure in the condensateheader 51.

It follows that the pressure in the common condensate header 51 is,using the figures quoted above, 415-67: 348 inches of water gauge aboveatmospheric pressure. However, due to the higher rate of condensation inthe first row of tubes than in the second row, the pressure drop acrossthe first row is 120 inches of water gauge and therefore in order toprovide the extra pressure across the first row so that the pressure atthe bottoms of the tubes of the first row is 348 inches of water gauge,a leg of condensate forms in the lower part of each tube of the firstrow. If the first row of tubes were supplied with a separate condensateheader, the pressure in the header would be 415-120=295 inches of watergauge above atmospheric pressure. Therefore, when the tubes of the firstrow ar connected to a condensate header having therein a pressure of 348inches of water gauge, the height of the leg of condensate in each tubeof the first row will be the difference between 348 inches Water gaugeand 295 inches water gauge, i.e. 53 inches water gauge. There will thusbe a condensate leg of 53 inches of Water in each tube of the first row.As described above, this condensate leg can be eliminated by ensuringthat the first row has a separate steam trap whereby the pressure dropacross the first row can be maintained independently of the pressuredrop across the second row of tubes.

In practice the Water column will be somewhat less than that calculatedsince it will reduce the effectiveness of heat transfer of the lowersections of the first row of tubes so that the steam required in thefirst row of tubes will be somewhat less than calculated above. Thesteam required in the second row of tubes will be somewhat more thancalculated above since the water in the bottoms of the first row oftubes will impose a greater load on the bottoms of the second row oftubes. However, it is believed that this does not affect the essentialconclusions of the above calculations which is that there will be adifference between the pressure drops in the tubes of the two rows dueto the difiering amounts of steam condensed and that this differencemust be equalized by a leg of condensate which may freeze and burst thetubes.

It will be seen that the invention provides a simple means of obviatinga heretofore undesirable and dangerous condition.

An important advantage of the invention is that the tubes of the firstand second rows can be connected to a common steam header so that thesteam pressure applied to-each row of tubes can be the same and nofreezing will occur since the pressure drop across each row of tubes isindependent of th pressure drop across the other row.

It will be understood that the form of the invention herewith shown anddescribed is a preferred example and that various modifications may becarried out without departing from the spirit of the invention or thescope of the appended claims.

What I claim as my invention is:

1. Apparatus for passing steam in heat exchange relationship with airhaving an initial temperature below the freezing point of water, saidapparatus comprising a plurality of vertically extending heat exchangertubes arranged in first and second rows, the second row being arrangedbehind the first row, means to propel a stream of air substantiallyhorizontally across the exterior surfaces of said tubes so that the airpasses across the tubes in the first row before passing across the tubesin the second row, means to supply steam to the top end of each of saidtubes for flow through the tubes, separate first and second condensateheaders, the first header being connected to the bottoms of the tubes ofthe first row and isolated from the second header which is connected tothe bottoms of the tubes of the second row, separate first and secondtraps to prevent escape of steam but to discharge condensate, firstconduit means interconnecting the first header and the first trap andsecond conduit means isolated from the first conduit means andinterconnecting the second header and the second trap.

2. Apparatus for passing steam in heat exchange relationship with aircomprising a plurality of heat exchanger tubes arranged in first andsecond rows, the second row being arranged behind the first row, acommon header connected to all said tubes at one end, separate first andsecond condensate headers, the first condensate header being connectedto the other ends of the tubes of the first row and isolated from thesecond condensate header which is connected to the other ends of thetubes of the second row, separate first and second traps to preventescape of steam but to discharge condensate, said first and second trapsbeing respectively and separately connected to said first and secondcondensate headers.

3. Apparatus for passing steam in heat exchange relationship with airhaving an initial temperature below the freezing point of water, saidapparatus comprising a plurality of vertically extending heat exchangertubes arranged in first and second rows, the second row being arrangedbehind the first row, means to propel a stream of air substantiallyhorizontally across the exterior surfaces of said tubes so that the airpasses across the tubes in the first row before passing across the tubesin the second row, a common header connected to the top ends of all ofsaid tubes, means to supply steam to said common header for fiow throughthe tubes, separate first and second condensate headers, the firstcondensate header being connected to the bottom ends of the tubes of thefirst row and isolated from the second condensate header which isconnected to the bottom ends of the tubes of the second row, separatefirst and second steam traps to prevent escape of steam but to dischargecondensate, first conduit means interconnecting the first condensateheader and the first trap and second conduit means isolated from thefirst conduit means and interconnecting the second condensate header andthe second trap.

References Qited in the file of this patent UNITED STATES PATENTS874,112 Peck Dec. 17, 1907 874,113 Peck Dec. 17, 1907 2,032,811 Perkinset al Mar. 3, 1936 2,217,410 Howard Oct. 8, 1940 2,238,688 Guler Apr.15, 1941 2,744,733 Howes May 8, 1956

1. APPARATUS FOR PASSING STEAM IN HEAT EXCHANGE RELATIONSHIP WITH AIRHAVING AN INITIAL TEMPERATURE BELOW THE FREEZING POINT OF WATER, SAIDAPPARATUS COMPRISING A PLURALITY OF VERTICALLY EXTENDING HEAT EXCHANGERTUBES ARRANGED IN FIRST AND SECOND ROWS, THE SECOND ROW BEING ARRANGEDBEHIND THE FIRST ROW, MEANS TO PROPEL A STEAM OF AIR SUBSTANTIALLYHORIZONTALLY ACROSS THE EXTERIOR SURFACES OF SAID TUBES SO THAT THE AIRPASSES ACROSS THE TUBES IN THE FIRST ROW BEFORE PASSING ACROSS THE TUBESIN THE SECOND ROW, MEANS TO SUPPLY STEAM TO THE TOP END OF EACH OF SAIDTUBES FOR FLOW THROUGH THE TUBES, SEPARATE FIRST AND SECOND CONDENSATEHEADERS, THE FIRST HEADER BEING CONNECTED TO THE BOTTOMS OF THE TUBES OFTHE FIRST ROW AND ISOLATE FROM THE SECOND HEADER WHICH IS CONNECTED TOTHE BOTTOMS OF THE TUBES OF THE SECOND ROW, SEPARATE FIRST AND SECONDTRAPS TO PREVENT ESCAPE OF STEAM BUT TO DISCHARGE CONDENSATE, FIRSTCONDUIT MEANS INTERCONNECTING THE FIRST HEADER AND THE FIRST TRAP ANDSECOND CONDUIT MEANS ISOLATED FROM THE FIRST CONDUIT MEANS ANDINTERCONNECTING THE SECOND HEADER AND THE SECOND TRAP.